Linear amplifiers as used in communication systems exhibit a linear characteristic where the amplifiers output power increases relative to an input power. After a certain input level, known as a gain saturation point, the amplifier then exhibits non-linear characteristics; the output power increases relatively little with respect to power input. This non-linear region is also known as the saturation region or the gain compression region.
In systems such as Code Division Multiple Access (CDMA) modulation schemes, a pluralit of signals are transmitted in a communication system and are amplified simultaneously. When a plurality of signals are applied to a linear amplifier, its non-linear characteristics will tend to produce interaction between the signals being amplified and the amplifier output will contain intermodulation products. Such intermodulation products reduce signal quality by allowing cross-talk to occur and such spillage often falls outside a particular licensed spectrum and must be controlled. Such intermodulation distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components with the amplifier output and feeding forward the distortion component to cancel the distortion of the amplifier output signal. Feed forward is more complicated in that it requires the modification of the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis.
WO97/24798 (Qualcomm) provides an out-of-band compensation system for a non-linear device. A signal source provided in the system provides an out-of-band signal, which out-of-band signal combines with a second signal derived from a non-constant envelope signal. The combined signal combines with the nonconstant envelope signal which is input to a nonlinear power amplifier. No feedback is employed; the power of the combined signal is maintained at a nominal operating point. U.S. PAT. NO. 4,885,551 (American Telephone and Telegraph Company) provides a feed forward linear amplifier. An input signal is applied to first and second paths one of which has a distortion component. The signals of the first and second circuits are combined to form a signal representative of the distortion component of the first circuit path whereby distortion can be cancelled. This complicated circuit however can be both difficult and expensive to produce.